A fuzzy set (class) A in X is characterized by a membership (charac- teristic) function fA(x) which associates with each point 2 in X a real number in the interval [0, 1], ~ with the value of fA(x) at x representing the "grade of membership" of x in A. Thus, the nearer the value of fA(x) to unity, the higher the grade of membership of x in A. When A is a set in the ordinary sense of the term, its membership function can take oil only two values 0 and 1, with fA(x) = 1 or 0 according as x does or does not belong to A. Thus, in this case fA(x) reduces to the familiar eharae~-eristic function of a set A. (When there is a need to differentiate between such sets and fuzzy sets, the sets with two-valued characteristic functions will be referred to as ordinary sets or simply sets. )
example. Let X be the real line R ~ and let A be a fuzzy set of numbers